Virtually nothing is known about the efficacy of stimulant drug treatment with hyperactive mentally retarded children. Research paralleling that established with non-retarded children is necessary to provide clinicians with dosage recommendations for prescribing and evaluating these medications as a part of treatment provided to hyperactive mentally retarded children. The participants will be 48 mentally retarded children having a DSM-III diagnosis of Attention Deficit Disorder - with Hyperactivity and an Abbreviated Conner's Teachers Rating Scale score of 15 or above. With those children aged 6-12 we will compare 0.3, 0.5, and 0.7 mg/kg methylphenidate and with those children aged 3-6 we will compare .15, .25 and .35 dextroamphetamine. The comparisons will be double-blind, placebo controlled, randomly counter-balanced, and assess changes in both targeted inappropriate behaviors and adaptive (academic, laboratory) behaviors. Data will be analyzed using within-group time series analyses as well as between-group analyses of variance/covariance with appropriate follow-up comparisons. The project will require two years to complete and will provide clinicians with valuable information with regard to the use of stimulant medications with mentally retarded children.